1. Field of the Invention
This invention relates to a zoom lens system for cameras, and more particularly to a zoom lens system for video cameras having electronic image pick-up devices as image sensors.
2. Description of the Related Art
In recent years, compact and lightweight video cameras have been offered at low prices, and the demand for video cameras for private use is increasing rapidly. In order to further develop this demand, it is necessary to design much more compact video cameras having lower weight and manufacturable at lower cost, thereby posing problems to obtain much more compact lens systems having lower weight and manufacturable at lower cost.
In general, it is difficult to design a compact, lightweight and inexpensive zoom lens system having a high zooming ratio and a large aperture ratio unless epoch-making inventions of optical glass material, etc., are made.
Therefore, as a solution, it has been tried to use a lens of a fixed focal length to minimize the camera, and such video cameras are on the market. However, a video camera using a lens system of a fixed focal length has the drawback that image variation is extremely limited.
Thus, attention is paid to zoom lens systems specified for a zooming ratio of about 2 to 3 and an aperture ratio of about F/2 to F/2.8 which are intermediates between the existing zoom lens systems of wide focal length variation ranges specified for a large aperture ratio of about F/l.2 to F/l.4 and a zooming ratio of about 6. Attention is also drawn to vari-focal lens systems having two focal lengths which can be changed over.
Examples of the vari-focal lens systems of this type are disclosed in Japanese Laid-Open Patent Applications, Publication Nos. 62-198813 and 63-49719. These vari-focal lens systems comprises a small number of lens components, that is, less than ten in total, are compact and lightweight, and have good imaging characteristics. However, the first lens unit, the diameter of which tends to be large, comprises a plurality of lens components and are large in diameter and heavy. In particular, it is not preferable to perform autofocusing, which is indispensable for modern cameras, by driving the heavy first lens unit because a large quantity of power is consumed and the response is delayed.
Further, in order to correct chromatic aberration properly over the focal length variation range of a zoom lens system, it is necessary to correct chromatic aberration to a certain extent in each lens unit. Therefore, conventional zoom lens systems have at least one positive lens component and at least one negative lens component in each lens unit, and the total number of lens components of the lens system is larger than ten so that other types of aberrations can be properly corrected. In many cases, the first lens unit also comprises a plurality of lens components.
Japanese Laid-Open Patent Applications, Publication Nos. 54-143127 and 63-85516 disclose zoom lens systems in which the first lens unit comprises a single lens and the second and third lens units are designed individually under certain conditions to overcorrect chromatic aberration so that the total chromatic aberration of the lens system can be corrected properly. The zoom lens system disclosed in Publication No. 54-143127 corrects chromatic aberration independently in the third lens unit, thus the third lens unit must comprise a plurality of lens components, and the zoom lens as a whole has nine lens components. The zoom lens disclosed in Publication No. 63-85516 overcorrects chromatic aberration individually in the second and third lens units, so that the total number of lens components contained in the zoom lens is eleven.